Gerard literally said that Allison's mother told him to read it. That she "wanted his thoughts", so, yeah, of course he knows what it says. He read it and talked to her as she was writing it. (though of course he could be lying and the letter could be fake, I don't remember if this is ever definitively answered)
Also, I don't think I've ever seen anyone not hate Gerard, he's universally despised, even moreso than Kate